Aqueous dispersions containing vinyl ester polymers are well known and are especially useful in the adhesive industry. For such uses, it is common to polymerize the monomer in an aqueous medium containing polymerization initiators and water soluble colloids. The most commonly used colloid is polyvinyl alcohol which has been hydrolyzed to a level of about 88%, i.e., which contains about 12% vinyl acetate. The resulting emulsion polymer is then formulated with either partially or fully hydrolyzed polyvinyl alcohol to obtain adhesive compositions having varying properties.
For many years, attempts have been made to prepare emulsion polymers which can be formulated with both partially and fully hydrolyzed polyvinyl alcohol and which also possess desirable adhesive properties with respect to wet tack and set speed and also have good machining properties. In particular, emulsion polymers prepared with the 88% hydrolyzed polyvinyl alcohol cannot be formulated with the fully hydrolyzed polyvinyl alcohol. This limits their use for certain end use applications where this stability is required.
Various approaches have been taken to overcome these problems. Most of these approaches have been based on the theory that raising the overall level of hydrolysis of the polyvinyl alcohol used in the polymerization of the emulsion polymer will promote improved compatibility of the emulsion when it is subsequently formulated with the fully hydrolyzed polyvinyl alcohol. In these cases, it has been proposed to use, as the polymerization colloid, polyvinyl alcohol which has been more highly hydrolyzed so as to so provide this formulation stability. Thus, in U.S. Pat. No. 3,827,996 issued Aug. 6, 1974 to Beresniewicz, the higher levels of hydrolysis were achieved by using 93 to 97% hydrolyzed alcohol or by blending fully hydrolyzed polyvinyl alcohol with sufficient amounts of other alcohols to produce the average of 5 to 7 mole percent vinyl acetate, provided the alcohols used in the blend did not have vinyl acetate levels above about 16 mole percent. Other formulations have been prepared using the same general approach but blending the polyvinyl alcohols so as to achieve a final level of only about 90 to 92% hydrolysis as was done in U.S. Pat. 4,521,561 issued Jun. 4, 1985 to Hausman et al. and by using blend of fully hydrolyzed polyvinyl alcohol with mixtures of different molecular weight partially hydrolyzed alcohols as in U.S. Pat. 5,143,966 issued Sep. 1, 1992 to Lee et al. Using these techniques which employ higher levels of hydrolysis in the polyvinyl alcohols used in the polymerization, the resulting emulsions do exhibit somewhat improved performance with respect to their compatibility with the fully hydrolyzed polyvinyl alcohol; however, in many cases, this improvement is coupled with a decrease in the performance or application properties such as mechanical stability, speed of set, as well as other machining properties such as ease of clean-up.
Regardless of the approach, it is seen that all the prior techniques for preparing improved emulsion polymers have stressed the necessity for using polyvinyl alcohol having relatively high levels of hydrolysis and have consistently avoided the use of alcohols having average vinyl acetate content greater than about 10 to 16 mole percent, believing that the use of such alcohols will jeopardize the compatibility with the fully hydrolyzed alcohol.